The present invention relates generally to disk drives, and more particularly to disk drives having an actuator servo control system incorporating an adaptive elliptic notch filter system to identify and attenuate resonant disturbances present in the actuator servo control system.
The seeking and tracking performance of disk drive actuator servo control systems is becoming evermore critical as disk track densities and spindle rotational velocities continue to increase. Unfortunately, one consequence of increased spindle rotatational velocities, along with decreases in both access times and transducer head flyheights, is an increase in resonant disturbances in drive actuator structures, resulting in even greater demands on the actuator servo control system.
An actuator resonant disturbance is a physical disturbance that vibrates, or xe2x80x9crings,xe2x80x9d around a particular constant or nearly constant frequency. Spectral analysis of a disk drive position error signal (PES) typically shows a complex resonant-disturbance structure involving multiple resonant disturbances in the PES. For instance, increased windage and spindle motor vibrations cause increased actuator E-block resonant disturbances (e.g., butterfly mode, M-mode, end-arm mode, etc.) and suspension resonances (e.g., torsional and bending). The presence of resonant disturbances in the PES results in reduced actuator servo-positioning ability and increased track mis-registration (TMR).
Therefore, to improve actuator servo control system performance and reliability, it is desirable to attenuate, or remove, the resonant disturbances from the PES. Conventional techniques for attenuating resonant disturbances in closed-loop servo systems utilize notch filters. A notch filter is a filter that can attenuate a resonant disturbance at a particular frequency, or in other words, xe2x80x9cnotch it outxe2x80x9d of the frequency spectrum. If more than one resonant disturbance frequency is present, multiple notch filters are placed in series with one another in the closed-loop servo system, with each filter xe2x80x9ctunedxe2x80x9d to attenuate a different resonant disturbance.
However, while conventional filtering techniques can be effective at attenuating resonant disturbances, the use of too many notch filters can actually create servo system instability. Servo-system stability is often measured by its phase-margin, with a minimum phase margin of approximately 30 to 35 degrees being satisfactory. A notch filter has the inherent characteristic of creating a phase-lag that results in a corresponding decrease in a servo system""s phase margin. Consequently, the more notch filters that are required, the smaller the servo system phase-margin becomes and the less stable the system becomes.
Furthermore, conventional filtering techniques are not adaptive to the dynamic nature of resonant disturbances. The frequencies of resonant disturbances are dependent on many factors, including temperature, and can vary under different disk drive operating conditions. Thus, while conventional filtering techniques might be effective at filtering resonant disturbances under one set of disk drive operating conditions, they can be ineffective under other conditions.
Thus, there is a need for a disk drive system having an actuator servo system with a resonant disturbance attenuation system that both minimizes phase-margin reduction and is adaptive to the varying resonant disturbance frequencies present during disk drive operation.
One aspect of the present invention provides a disk drive and a method of attenuating resonant disturbance frequencies present in the disk drive actuator servo control system. The disk drive includes a servo control system and an adaptive resonant disturbance attenuator. The servo control system includes at least one disk having a plurality of radially-situated data tracks, an actuator structure, and a servo compensator. The actuator structure includes at least one transducer head for writing and/or reading data from the disk data tracks, a flexible suspension, an arm structure, and a voice coil motor, and provides a transducer head position error signal (PES). The servo compensator applies a servo-control algorithm to the PES to thereby produce a servo-control signal. The adaptive resonant disturbance attenuator is adapted to analyze the PES for a transducer head at a data track radii and identify resonant disturbances present in the PES. The adaptive resonant disturbance attenuator is further adapted to select two adjacent resonant disturbances having a first and a second resonant frequency, and to ascertain the coefficients of at least a fourth-order elliptic notch filter transfer function having a first zero-notch with a frequency substantially at the first resonant disturbance frequency and a second zero-notch with a frequency substantially at the second resonant disturbance frequency. The adaptive resonant disturbance attenuator is adapted to apply the ascertained coefficients to the at least fourth-order elliptic notch filter transfer function to thereby generate a tuned fourth-order digital elliptic notch filter and to apply the tuned fourth-order digital elliptic notch filter in the servo system to thereby substantially attenuate the selected adjacent resonant disturbances.
In one embodiment, the adaptive resonant disturbance attenuator analyzes the position error signals for a plurality of transducer heads at one or more selected data track radii and generates a plurality of tuned fourth-order digital elliptic notch filters to thereby substantially attenuate a plurality of selected resonant disturbances associated with each transducer head at each selected data track radii.
In one embodiment, the adaptive resonant disturbance attenuator applies the tuned fourth-order digital elliptic notch filter to the PES to thereby substantially attenuate from the PES the selected adjacent disturbances. In one embodiment, the adaptive resonant disturbance attenuator applies the attenuator tuned fourth-order digital elliptic notch filter to the servo control signal to thereby substantially attenuate from the servo control signal the selected adjacent resonant disturbances.
In one embodiment, the adaptive resonant disturbance attenuator includes a spectral disturbance frequency analyzer, a notch filter coefficient controller and a tunable digital elliptic notch filter. In one embodiment, the spectral disturbance frequency analyzer measures the PES for a transducer head at a data track radii, computes form the PES a non-repeatable position error signal (NRRO), and computes from the NRRO a spectrum of estimated resonant disturbance frequencies present in the NRRO. In one embodiment, the spectral disturbance frequency analyzer computes from a limited number of frequency bands within the NRRO the spectrum of estimated resonant disturbance frequencies.
In one embodiment, the notch filter coefficient controller analyzes the spectrum of estimated resonant disturbance frequencies and estimates the frequencies of resonant disturbances present in the spectrum of disturbance frequencies. The notch filter coefficient controller selects two adjacent resonant disturbances having a first and second resonant frequency and ascertains coefficients of a fourth-order elliptic notch filter transfer function having a first zero-notch with a frequency substantially at the frequency of the first selected resonant disturbance frequency and a second zero-notch with a frequency substantially at the frequency of the second selected resonant disturbance frequency. In one embodiment, the ascertained coefficients are stored in a memory.
In one embodiment, the tunable digital elliptic notch filter circuit receives the ascertained coefficients from the notch filter coefficient controller and applies the coefficients to a fourth-order elliptic notch filter transfer function to thereby generate a tuned fourth-order digital elliptic notch filter having a first zero-notch with a frequency substantially at the first selected resonant disturbance frequency and a second zero-notch with a frequency substantially at the second selected resonant disturbance frequency. The tuned fourth-order digital elliptic notch filter is then applied in the servo system to thereby substantially attenuate the selected adjacent resonant disturbances.
One aspect of the present invention provides a method for attenuating resonant disturbances in a disk drive comprising at least one disk having a plurality of radii and an actuator servo control system including a servo compensator and an actuator structure having at least one transducer head for writing and/or reading data form the disk surface, a flexible suspension, an arm structure and a voice coil motor, and providing a transducer head position error signal (PES). The method includes analyzing the PES for a transducer head at a disk radii, identifying resonant disturbances present in the PES, and selecting at least two adjacent resonant disturbances having a first and second resonant frequency. The method further includes ascertaining coefficients of at least a fourth-order digital elliptic notch filter transfer function having a first zero-notch with a frequency substantially at the first resonant disturbance frequency and a second zero-notch with a frequency substantially at the second resonant frequency, and applying the at least fourth-order digital elliptic notch filter transfer function with the ascertained coefficients in the servo system to thereby substantially attenuate the selected adjacent resonant disturbances.
A disk drive according to the present invention provides more effective attenuation of resonant disturbances present in disk drive servo systems by performing a continuous in situ analysis of transducer head position error signals (PES) to identify dynamic resonant disturbance frequencies and by tuning digital elliptic notch filters to attenuate the identified resonant disturbance frequencies. Adaptive tuning of digital elliptic notch filters to match dynamic resonant disturbance frequencies provides more effective attenuation of resonant disturbances than use of static filters tuned to match predetermined resonant disturbances. Additionally, by utilizing digital elliptic notch filters, the present invention improves servo system stability by preserving the servo system""s phase-margin.